This invention concerns a process for the separation of aromatic hydrocarbon mixtures into single components.
The ortho, para, metaxylene and ethylbenzene C.sub.8 isomers, in condition, are used industrially as raw materials for the production of various petrochemicals which, in turn, are used to prepare plastics, resins and other things. It is well known that the separation of the individual isomers from their mixtures is a very difficult operation, because of the proximity of the boiling points and because of the similarity of the other chemical-physical characteristics.
More recently, the separation of these isomers fulfilled has been accomplished through selective adsorption on zeolite-type materials. Such a separation can be theoretically considered as a two-stage process: an adsorption stage, where a part of the components of the C.sub.8 fraction is fixed on the zeolite material, thus undergoing a first separation; and a desorption stage, where the product fixed on the zeolite is separated by adding another component (desorbent) which has a greater affinity for the zeolite compared to that of the adsorbed material. Of the two steps, the desorption one is usually slower and therefore affects the rate of the total process.
In U.S. Pat. No. 3,773,846 there is disclosed a process for the separation on zeolite of meta- and paraxylene from a liquid fraction of C.sub.8 aromatic compounds. In such a process the desorbent used is any one of the xylene fractionation products, preferably toluene or diethylbenzene. It is important to emphasize that in the process described in that patent, the products used as desorbent are reputed to be fully equivalent and accordingly are not important. However, as will be pointed out below, toluene is not especially suitable to desorb the components being fixed on the zeolite. Ethylbenzene cannot in turn be separated from the other isomers.
Hence, there is no need to take advantage of a complicated series of traditional operations, such as fractional crystallization and so on.
In Italian patent application No. 24044 A/80 there is disclosed the use of type Y zeolite material having large pores exchanged with potassium.
The so treated materials absorbs the paraxylene very well and the process uses toluene as desorbent.
The desorption by means of this type of aromatic however, is rather slow. For the purpose of the above-mentioned application which is basically to separate a pure metaxylene vapor fraction, toluene is more suitable since it has affinity for the zeolite that is intermediate between that of the metaxylene and that of the ethylbenzene ones. Paraxylene however, is hardly displaced by toluene. Therefore, there must be used large quantities of toluene during the desorption process to make the removal complete.